The present invention relates generally to characterizing porous materials and more particularly to a method for characterizing nanoporous materials.
In the manufacture of integrated circuits, the dimensions of devices are scaled downwards in order to make them smaller and faster. As the dimensions further decrease, many formerly insignificant sources of error become magnified and must be monitored and compensated for. Of particular concern is the porosity of low dielectric constant (low-k) and ultra-low-k dielectric materials used as insulators in the integrated circuits.
Determining and compensating for porosity is important because it influences several significant problems. For example, to much porosity in a dielectric later can result in poor interfaces with subsequently deposed layers. Such poor interfaces can result in electromigration, movement of atoms associated with a current flow, and the formation of voids. Voids have high electrical resistance and nucleate, growing larger and eventually resulting in open circuits, which can render an integrated circuit inoperable.
Another example is in the conformal deposition of subsequent layers. In older, larger integrated circuits, the porosity of the dielectric layers did not affect the conformal deposition of subsequent layers. With the smaller devices and similarly scaled down layers that compose them and their interconnections, porosity becomes a factor, sometimes resulting in non-conformal surfaces, which can adversely affect the performance of the integrated circuit.
In addition, greater porosity makes a dielectric layer more prone to diffusion of conductor material. This is particularly common in the interconnects which connect semiconductor devices where diffusion allows conductor material to leak through the dielectric layer, eventually creating a short circuit.
Porosity, the size and interconnection of pores, can also influence the dielectric constant of the dielectric material. Because such dielectric materials are selected specifically for their dielectric constants, being able to measure the pore size and pore connectivity in the material allows for a better characterization of a material.
Also, it is extremely difficult but important to determine the shapes of the pores. Non-spherical cores have relatively sharp corners, which create electrical stress concentrations and physical stress concentrations, which lead to material failures.
Many of the above problems can be remedied or compensated for if the porosity of the dielectric material is known to a precise degree. Prior attempts to measure the porosity, pore size and pore interconnection for low-k dielectric layers have utilized scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to create high-resolution images of the cross-sections. While both technologies are relatively easy to implement and are capable of creating very small images, they have proved inadequate for characterizing nanoporous materials. Nanoporous materials are materials which have microstructures with a physical order in the sub 1 E-9 meter range, such as well-known commercially available materials such as Spin On Glass (SOG), porous silicas, organic dielectric materials (such as porous SILK, a material from Dow Chemical of Midland, MI), porous LDK materials (from JSR Microelectronics, Inc., of Sunnyvale, Calif.), or organic doped silicas. The ability to simply and inexpensively characterize the pore size, interconnections, and shape for these nanoporous materials is of particular concern because currently, only methods requiring powerful and expensive equipment are adequate.
For example, Rutherford Backscattering Spectroscopy (RBS) and Positron Annihilaton Lifetime Spectroscopy (PALS) have been used in the past to measure pore size and pore connectivity, but each of these methods requires extra equipment not normally found in semiconductor fabrication plants. For example, RBS is not used in a fabrication environment because it requires an energy source to create an ion beam, which impacts the surface of the porous material. A special Rutherford Backscattering detector is then required to measure the intensity of backscattered particles. PALS works by using the variation in positron lifetimes as a function of the electron density in a material but also requires special, expensive equipment not commonly available.
Because current methods are either inadequate or inconvenient to implement, a simple method for characterizing the porosity of nanoporous materials in an inexpensive manner using commonly available equipment has been long sought but has long eluded those skilled in the art.
The present invention provides a method for measuring porosity of nanoporoos materials. Using atomic force microscopy (AFM), a surface topology map with subatomic resolution is created wherein the pore shape and size can be determined by measuring the pores that intersect the top or fracture surface. For porous materials requiring more accurate measurements, small scan areas with slow scan speed and fine AFM tips are used and a general estimation on distribution is made from a sample areand
The above and additional advantages of the present invention will become apparent to those skilled in the art firm a reading of the following detailed description when taken in conjunction with the accompanying drawings.